You have a very good point. Seems to me happened is that the definition of "genocide" connected to a specific "people" has its roots in the definition of other war crimes. The concept became especially prominent after WWII. Before that I don't think such a distinction was so firmly rooted in people's minds. Sure, it's a way to enshrine the historical importance of the Nazi extermination of the Jews, and it might have served as a way to more easily justify the UN/"peacekeeping" troops inserting themselves in the middle of certain conflicts such as the ones in Yugoslavia or in Rwanda.

It supposedly protects distinct cultural groups from being destroyed which is the right thing to do in today's definition. However, as Sapiens put forward, empires throughout the history have assimilated all sorts of people, and objectively speaking, they didn't necessarily made the lives of those people worse, and in many cases voluntary bi-directional cultural influence took place, until one is barely distinguishable from another. Therefore, I agree to some extent that it seems weird to elevate this definition of "genocide", even sometimes cultural, as something graver than the act of actual mass murder. Sometimes they do happen together, to some extent even in this example, where the children died en masse. But in many occasions throughout the history, the gradual disappearance of a cultural identity is a very distinct thing than people actually dying or having their living conditions worsening. As you said, group identities disappear and reappear in an endless ebb and flow, maybe not so much in the modern world with all these nation borders, but much more commonly throughout the history.

Moreover, a deliberate campaign to eradicate another group's identity, is now something that can be carried out by PR professionals. It puts into perspective that the most powerful companies in the world are in the business of advertising. Or, to use provocative language that is literally true: mind control.